Abstract: Most current microbial pathogenesis studies are focused on understanding how different virulence factors may enable pathogens to disturb and invade the normal flora. This study focuses on ?the other side of the story?: how normal flora may prevent and resist the invasion of foreign pathogens as the first line of defense. Understanding the mechanisms underlying the invasion resistance of host flora against foreign invading bacteria, including pathogens, is of great scientific and clinical importance, which is the main goal of this application. During our preliminary studies, we established an in vitro model system for studying the community-level invasion resistance against the colonization of bacteria of different origin, using oral and gut microbial communities derived from mice. Specifically, we identified a unique microbial consortium composed of three distinct bacterial species within murine oral microbial flora that were involved in restricting invasion of gut- derived Escherichia coli. Further studies revealed that these three species form a unique social structure; they act as the sensor, the mediator and the killer, respectively, and have coordinated roles in preventing the integration of E. coli into oral microbial communities. Our data implicated sophisticated signaling events when the consortium was challenged with the invading species. Most intriguingly, we demonstrated that a similar invasion resistance phenomenon exists in the human oral microbial community as well. Based on these findings, we hypothesize that indigenous microbiota-based invasion resistance against foreign bacteria is a highly coordinated community-level function, which involves extensive molecular events for recognition of foreign invaders, inter-species communication and regulated production of killing molecules. Understanding these molecular events will provide valuable insights into the protective effect conferred by normal flora. We recognize the fact that a murine microbiota study will serve as an ideal model system for investigating the detailed molecular mechanisms as well as laying a foundation for future animal studies, and that a human microbiota study is critical for determining the clinical relevance of the observed invasion resistance mechanism, therefore, this application investigates this intriguing phenomenon in both systems. The two main goals of this application are: 1) To further understand the molecular mechanism of invasion resistance using the well established three-species consortium model system derived from murine oral flora; 2) To address the clinical relevance of the observed protective mechanism by expanding the study to human oral microbiota. The results of these studies will provide insightful mechanistic information on how the normal flora may resist the invasion of foreign pathogens at the molecular level. This critical information could lead directly to novel therapeutic approaches that seek to enhance the protective efforts of normal flora.